The major emphasis of this project has been the application of recombinant DNA technology to the construction of molecular clones for C-type murine retroviruses, primate retroviruses, and mammalian structural genes. These reagents are to be used for the molecular analysis of mechanisms involved in viral tumorogenesis and eucaryotic gene regulation. Recombinant clones have been isolated and characterized for unintegrated circular intermediates of Harvey Sarcoma Virus (HaSV), Friend Murine Leukemia Virus (FrMuLV), and Baboon Leukemia Virus (BLV), for the integrated copies of AKR Murine Leukemia Virus, and for cellular genes expressed in human reticulocytes. For the C-type retroviruses, the cloned DNA has been shown to be biologically active, either in a transformation assay (HaSV) or in a XC plaque assay (FrMuLV and AKR). Virus isolated from cells transfected with FrMuLV clone DNA produces rapid nonthymic leukemia. Restriction and hybridization analysis of the clones expressed in reticulocytes reveals repetitive sequences interspersed between unique copy sequences.